1. Field of the Invention
The present invention relates to a microphone connector, and specifically, a microphone connector having a structure that effectively prevents a high-frequency current from entering the microphone through a cable.
2. Related Background Art
A capacitor microphone includes a microphone unit having extremely-high output impedance and, thus, accommodates an impedance converter including a FET (Field-Effect Transistor) that reduces impedance to output audio signals. The audio signals are output through a connector and a cable to the exterior. Normally the capacitor microphone includes a phantom power supply to output the audio signals electro-acoustically converted by a microphone through a balanced-shielded cable for the phantom power supply. That is, a microphone cable is composed of the balanced-shielded cable. A microphone casing or a handheld microphone has a three-pin microphone connector in a grip to be connected to the balanced-shielded cable. The connector, generally specified in ETAT RC-5236 “Latch Lock Type Round Connector for Audio Equipment”, is widely known.
The connector provides a connector base composed of an electrical insulator such as PBT (polybutylene terephthalate). A first pin for grounding, a second pin for the hot side of signals, and a third pin for the cold side of the signals penetrate through the connector base. A connector of a handheld microphone, for example, is mounted in a cylindrical connector casing threadably fixed to the end of a microphone grip. Commonly used microphone grips and cylindrical connector casings are composed of a metallic material such as brass. The microphone grip also functions as a shield cover for inner electrical components. The connector base has a female thread into which a male thread is screwed. The male thread electrically connects the first pin for grounding to the cylindrical connector casing.
If such a connector is connected to the balanced-shielded cable drawn from the phantom power supply and the microphone and the microphone cable are subjected to strong electromagnetic waves, the electromagnetic waves may enter the microphone through the connector to be demodulated and output from the microphone as noise having an audible frequency.
To address this circumstance, a microphone connector is proposed which includes a connecter base covered with a shield cover on at least one surface, as is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2005-094575. The shield cover is electrically connected to the first pin for grounding, not to the second and third pins for signals.
Japanese Unexamined Patent Application Publication No. 2005-311752 discloses a microphone connector including a printed circuit board disposed on one surface of a connector base through which three pins penetrate and a shield cover through which these three pins penetrate and is disposed to cover the printed circuit board and the connector base. In the invention disclosed in Japanese Unexamined Patent Application Publication No. 2005-311752, the printed circuit board mounts a capacitor element for preventing entrance of high-frequency electromagnetic waves and a zener-diode element for preventing destruction of circuits due to static electricity, and the surface of the printed circuit board on which the capacitor element and the zener-diode element are disposed is covered with the shield cover.
These patent documents describe that high-frequency electrical currents can be prevented from entering the microphone through a connector.
Recent achievement of widespread use of, for example, cellular phones increases radiation of electromagnetic waves having higher frequencies in close proximity to microphones. Disadvantageously, this generates noises caused by the electromagnetic waves. In the inventions disclosed in the patent documents, however, noises caused by electromagnetic waves having extremely high frequencies and emitted in close proximity to microphones cannot be sufficiently shielded. It is needed to still further increase a shielding effect of the shield cover. Furthermore, in the invention disclosed in Japanese Unexamined Patent Application Publication No. 2005-311752, a ground pattern of the printed circuit board and a shield cover are electrically connected only by mechanically connecting the shield cover to the pin for grounding. Accordingly the shielding effect against the electromagnetic waves having high frequency cannot be achieved.